Evolution of mechanical and electrical properties of tin–lead and lead free solder to copper joint interface

Abstract

Cu–(Sn37Pb) and Cu–(Sn3.5Ag0.5Cu) solder joints were prepared at the same reflow temperature of 230 °C. The microstructural observation of the solder assemblies in scanning and transmission electron microscopes confirmed the presence of η-Cu6Sn5 in case of the former, and Cu3Sn + η-Cu6Sn5 for the latter in the reaction zone. The findings are correlated with the electrical and mechanical properties of the joints. Lead free solder-Cu joint exhibited lower reaction zone thickness and improved electrical conductivity (0.28 × 106Ω− 1 cm− 1) and shear strength 68MPa compared to conventional lead–tin solder-Cu joint. The latter showed electrical conductivity and shear strength of 0.22 × 106Ω− 1 cm− 1 and 55 MPa, respectively. The difference in reaction zone thickness is explained on the basis of melt superheat, with Sn being the primary diffusing species in the intermetallic layer.